Reductive method for production of metallic elements such as chrome using a crucible with a perforated wall

ABSTRACT

In the method of producing metals or metal alloys of high purity, in particular metallic chromium, granules of metal containing non-metallic inclusions and a reducing agent are treated under predetermined conditions of temperature and pressure so that the reducing agent reacts on the inclusions. During the treatment, the granules ( 26 ) are placed in a crucible ( 2 ) having an opening ( 14 ), and a wall ( 4 ) presenting at least one orifice ( 24 ).

The invention provides a method of reducing metals or metal alloys ofhigh purity, and in particular metallic chromium.

Certain industries require metals and metal alloys of ever increasingpurity. This applies in particular to aviation industries forfabricating the noble parts of turbojets.

In document EP-0 102 892, the Applicant discloses a method of producingmetals or alloys comprising the steps consisting in:

a) preparing a metal or a metal alloy in which the non-metallicinclusions are essentially oxides of the base metal;

b) grinding the resulting metal or metal alloy and mixing it with apelletizing agent and a reducing agent to form pellets; and

c) subjecting the pellets to a vacuum reducing treatment underconditions of pressure and temperature that are controlled so that thereducing agent reacts on the non-metallic inclusions and so that thereis no significant sublimation of the metal or of the alloy metals beingtreated.

That method can involve, in particular, an aluminothermic reaction instep a), said reaction being unbalanced by a shortage of aluminumrelative to the quantity needed for a complete reaction. That methodenables high purity metallic chromium to be obtained.

Nevertheless, the relative proportion of some impurities can still betoo high for some uses of the metal or the alloy. This applies inparticular to the contents of atoms of carbon, nitrogen, and oxygen.

An object of the invention is to further improve the purity of the finalproduct.

To this end, the invention provides a method of producing granules ofmetal in which granules of metal containing non-metallic inclusions anda reducing agent are treated under predetermined conditions oftemperature and pressure so that the agent reduces the inclusions, andin which, during the treatment, the granules are disposed in a cruciblehaving an opening and a wall presenting at least one orifice.

The Applicant has found that the presence of one or more orifices in thecrucible improves the purity of the final metal or alloy. This appliesin particular for atoms of oxygen and carbon for which it has beenpossible to reduce the relative concentrations on average by 56% and 70%respectively in the samples that the Applicant has analyzed.

Preferably, the crucible is made for the most part out of graphite, orentirely out of graphite.

Here also, the Applicant has found, surprisingly, that contrary to thatwhich might have been expected, the granules are not polluted by thecarbon forming the graphite, and that on the contrary such a crucibleenables the purity of the product to be increased.

The method of the invention may also present at least one of thefollowing characteristics:

-   -   the wall is a side wall;    -   a majority of the orifices occupy a bottom half of the wall;    -   the orifices occupy the bottom two-thirds of the wall;    -   the orifices are disposed in such a manner that more than half        the total area defined by the sum of the areas of the orifices        occupies the bottom half of the wall;    -   the wall is free from orifices over at least a top-fourth of its        height from the opening;    -   the ratio of the total area of the orifice(s) over the total        inside volume of the crucible lies in the range 0.5 to 1.5, and        preferably in the range 0.80 to 1.20;    -   the or each orifice has an area lying in the range 50 square        millimeters (mm²) to 150 mm², and preferably in the range 90 mm²        to 130 mm²;    -   the orifices are mutually identical;    -   the crucible is of generally constant shape;    -   the crucible is generally circularly symmetrical in shape;    -   the crucible is cylindrical in shape;    -   the treatment is performed under a partial vacuum;    -   during the treatment, the granules are subjected to an air flow;    -   the granules are constituted by a metal such as chromium,        titanium, vanadium, molybdenum, manganese, niobium, tungsten,        and nickel, or an alloy comprising one of those metals and boron        or iron;    -   prior to the treatment, a metallic compound is prepared by means        of an aluminothermic reaction between at least one metallic        oxide and divided aluminum, and the granules are made from said        compound;    -   prior to treatment, the granules are baked; and    -   the method is implemented to produce metallic chromium.

The invention also provides a crucible for producing metallic granules,the crucible possessing an opening and having a wall presenting at leastone orifice.

Other characteristics and advantages of the invention appear furtherfrom the following description of a preferred implementation given byway of non-limiting example. In the accompanying drawing, the soleFIGURE is an axial vertical section view of a crucible constituting apreferred embodiment of the invention.

The description begins with the crucible of the invention. Thereafterthe method in which the crucible is implemented is described.

The crucible 2 comprises a vertical side wall 4 of generally circularcylindrical shape about an axis 6. The shape of the wall is thusessentially constant along the axis 6, the wall presenting a sectionthat is circular in a plane perpendicular to the axis. The wall 4presents an outside face 8 that is accurately cylindrical in shape andan inside face 10 that is slightly frustoconical in shape, tapering alittle, with the axis 6 constituting the axis of the cone and with theapex of the cone pointing downwards. The diameter of the inside face 10thus decreases going downwards.

The wall 4 presents a circular top edge 12 of plane shape defining a topopening 14 of the crucible.

The crucible has a flat bottom 16 closing a bottom axial end of the wallremote from the opening 14. At the junction between the outside face 8of the wall 4 and the bottom face 18 of the bottom 16, the cruciblepresents a circular shoulder 20 recessed into these two faces and givingthe bottom face 18 a diameter that is slightly smaller than that of theopening 14 so as to enable two crucibles to be engaged one in anotherwhen they are stacked.

In its top third, the outside face 8 is recessed by a peripheral groove22 of channel section making the crucible easier to handle with a tool.

The crucible is made of graphite.

The side wall 4 in this example presents a multitude of orifices 24passing through the thickness of the wall so as to put the inside of thecrucible into communication with the outside. Only some of the orificesare shown in FIG. 1. Specifically, the orifices are disposed in aplurality of circular horizontal rows, each row occupying a planeperpendicular to the axis 6. In this example, there are 14 such rows.Each row has 20 orifices uniformly distributed around the circumferenceof the wall. The rows follow one another, being spaced apart by the samedistance. The orifices in successive rows are disposed in a staggeredconfiguration, each orifice of a given row forming an isosceles trianglewith the nearest two orifices in the row above and/or the row below. Therows follow one another uniformly. They are disposed in such a mannerthat the orifices occupy the bottom two-thirds of the height of the wall4, the top-third adjacent to the opening 14 being completely free fromany orifices.

By way of example, the dimensions of the crucible are as follows:

-   -   total height, 516 millimeters (mm);    -   height of the crucible from the opening 14 to the inside face of        the bottom 16, 476 mm;    -   total diameter of the crucible, 360 mm;    -   inside diameter of the opening, 313 mm, inside diameter of the        bottom, 288 mm;    -   outside diameter of the crucible at the bottom of the groove 22,        344 mm;    -   the groove 22 is 100 mm from the top edge 12;    -   the height of the groove is 60 mm;    -   the highest row of orifices is 20 mm below the groove 22,        measured to the plane passing through the centers of the        orifices.

Using identical references for each row, the rows follow one belowanother at a spacing of 20 mm. The bottom row is thus about 30 mm fromthe bottom. Given the thickness of the wall 4, the orifices in this caseform ducts, and specifically they have a diameter of 12 mm. The orificesare identical to one another. The area of each orifice is about 113 mm².Since the number of orifices in this case is 280, the total area of theorifices, i.e. the sum of their individual areas, is about 0.0317 squaremeters (m²). The total inside volume of the crucible is about 0.336cubic meters (m³). The ratio of the total area of the orifices over thetotal volume of the crucible is thus about 0.94 in this case.

There follows a description of how the method of the invention isimplemented with the above-described crucible in order to producemetallic chromium.

Step a

Chromium oxide (Cr₂O₃), potassium bichromate (K₂Cr₂O₇) and dividedaluminum are introduced into an ordinary crucible. The chromium oxideand the potassium bichromate are present in proportions appropriate forthe aluminothermic reaction. The aluminum is present with a shortagerelative to the proportion required for complete reaction. This shortagemay lie in the range 0.5% to 8%, or indeed 2% to 5% by weight of thestochiometric quantity.

These three ingredients are mixed and then the reaction is initiated. Atthe end of the reaction, the metal is collected from the bottom of thecrucible. The elemental chromium is reduced and the resulting finalproduct is metallic chromium of high purity identical to thealuminothermic chromium that would have been obtained with a completereaction, except that it contains a very high oxygen content, whichoxygen is almost exclusively present in the form of non-metallicinclusions of Cr₂O₃ (0.40% to 0.80% or even more) together with very fewalumina inclusions Al₂O₃ (100 parts per million (ppm) to 400 ppm,corresponding to 50 ppm to 200 ppm of oxygen bonded with aluminum).Consequently, metallic chromium is obtained with non-metallic inclusionsthat are constituted mainly by inclusions of Cr₂O₃ that can easily beeliminated, and to a minor extent by inclusions of alumina that are moredifficult to eliminate, but that are present in smaller quantity.

Step b

The chromium from step a) is ground in an impact grinder so as to obtaina fine powder that passes through the screen with a mesh size of 500micrometers (μm). The grinder bursts these grains, thereby releasing agood fraction of the non-metallic inclusions of Al₂O₃ and Cr₂O₃, withthe Cr₂O₃ inclusions appearing to be released preferentially. Thisgrinding is purifying and produces an air flow. The air flow may also beproduced by an auxiliary device such as a blower which contributes toexhausting into ambient air some of the non-metallic inclusions thathave been released. A screening step performed at this stage can serveto remove another fraction of the inclusions.

The resulting purified chromium powder is then mixed intimately with areducing agent and a pelletizing agent. By way of example, thepelletizing agent may be a mixture of Bakelite and an organic bindersuch as furfuraldehyde. The reducing agent may be constituted by carbonblack.

The resulting mixture is formed into pellets or tablets using aconventional compacting press.

After being formed into pellets, the mixture is baked at an appropriatetemperature (e.g. 200° C. to 230° C.).

Step c

The resulting pellets 26 are then placed in the crucible 2 and subjectedto reducing treatment at 1100° C. to 1400° C. under a vacuum of about133×10⁻⁴ pascals (Pa) The crucible is filled with pellets up to itsopening.

At the beginning of the vacuum heating cycle, the Bakelite decomposes ata certain temperature, leaving a carbon skeleton which adds to thecarbon black that was introduced into the mixture as a reducing agent.Once the treatment temperature has been reached, this carbon reacts withthe oxygen of the Cr₂O₃ that remains in the material, but reacts hardlyat all with the oxygen of the alumina Al₂O₃.

The vacuum in the treatment furnace is brought to 133×10⁻¹ Pa bycontrolled sweeping with a non-oxidizing gas or a reducing gas such ashydrogen. To terminate, the product is allowed to cool under an inertatmosphere.

The presence of the orifices appears to have a great influence on thecontents of certain impurities, and in particular of oxygen and carbonatoms. The Applicant has undertaken experiments, treating pellets havingthe same composition in crucibles that are not pierced and in cruciblesthat are pierced. The contents of atoms of oxygen, of nitrogen, and ofcarbon were analyzed in the final products, and these contents aresummarized in the table below: O₂ C N Non-pierced (ppm) 852 450 31Pierced (ppm) 376 135 24 Difference (%) −56 −70 −22

The impurity contents are given in parts per million (ppm) while thedifference is given as a percentage. It can be seen that the presence oforifices enables the content of oxygen atoms to be reduced by about 56%and the content of carbon atoms by about 70%.

It is probable that the presence of the orifices facilitates gas flowthrough the crucible during treatment, the orifices co-operating withthe opening 14 to cause the gas to flow over the full height of thecrucible.

It is preferable to provide no orifices in the top portion of thecrucible in order to avoid weakening the crucible.

Naturally, numerous modifications can be applied to the inventionwithout going beyond the ambit of the invention.

The crucible presenting orifices may be made out of a material otherthan graphite. A graphite crucible could be provided that does not haveany orifices other than the opening.

The orifices need not be disposed uniformly in the wall. The orificescould be of differing sizes.

Similarly, step a) could be undertaken other than by aluminothermically,for example silicothermically or by reducing in an electric furnace, inorder to obtain a metal or a an alloy having non-metallic inclusions inthe form of oxides of the base metal.

1. A method of producing granules (26) of metal in which granules ofmetal containing non-metallic inclusions and a reducing agent aretreated under predetermined conditions of temperature and pressure sothat the agent reduces the inclusions, the method being characterized inthat during the treatment, the granules (26) are disposed in a crucible(2) having an opening (14) and a wall (4) presenting at least oneorifice (24).
 2. A method according to claim 2, characterized in thatthe wall is a side wall.
 3. A method according to claim 2, characterizedin that a majority of the orifices (24) occupy a bottom half of the wall(4).
 4. A method according to claim 2 or claim 3, characterized in thatthe orifices (24) occupy the bottom two-thirds of the wall.
 5. A methodaccording to any one of claims 2 to 4, characterized in that theorifices are disposed in such a manner that more than half the totalarea defined by the sum of the areas of the orifices occupies the bottomhalf of the wall (4).
 6. A method according to any one of claims 2 to 5,characterized in that the wall (4) is free from orifices over at least atop-fourth of its height from the opening.
 7. A method according to anypreceding claim, characterized in that the ratio of the total area ofthe orifice(s) (24) over the total inside volume of the crucible lies inthe range 0.5 to 1.5, and preferably in the range 0.80 to 1.20.
 8. Amethod according to any preceding claim, characterized in that the oreach orifice (24) has an area lying in the range 50 mm² to 150 mm², andpreferably in the range 90 mm² to 130 mm².
 9. A method according to anypreceding claim, characterized in that the orifices (24) are mutuallyidentical.
 10. A method according to any preceding claim, characterizedin that the crucible (2) is at least for the most part made of graphite,and is preferably made entirely of graphite.
 11. A method according toany preceding claim, characterized in that the crucible (2) is ofgenerally constant shape.
 12. A method according to any preceding claim,characterized in that the crucible (2) is generally circularlysymmetrical in shape.
 13. A method according to any preceding claim,characterized in that the crucible (2) is cylindrical in shape.
 14. Amethod according to any preceding claim, characterized in that thetreatment is performed under a partial vacuum.
 15. A method according toany preceding claim, characterized in that during the treatment, thegranules (26) are subjected to an air flow.
 16. A method according toany preceding claim, characterized in that the granules (26) areconstituted by a metal such as chromium, titanium, vanadium, molybdenum,manganese, niobium, tungsten, and nickel, or an alloy comprising one ofthose metals and boron or iron.
 17. A method according to any precedingclaim, characterized in that prior to the treatment, a metallic compoundis prepared by means of an aluminothermic reaction between at least onemetallic oxide and divided aluminum, and the granules are made from saidcompound.
 18. A method according to any preceding claim, characterizedin that prior to treatment, the granules (26) are baked.
 19. A methodaccording to any preceding claim, characterized in that it isimplemented to produce metallic chromium.
 20. A crucible (2) forproducing metal granules, the crucible presenting an opening (14) andhaving a side wall (4), the crucible being characterized in that theside wall presents at least one orifice (24).